# Solution with and without presolve is different - can this behavior be expected?

**URL:** <https://discuss.ampl.com/t/solution-with-and-without-presolve-is-different-can-this-behavior-be-expected/335>\
**Category:** Support\
**Tags:** general, solvers, displays-results\
**Created:** [January 23, 2023, 2:47pm UTC](https://discuss.ampl.com/t/solution-with-and-without-presolve-is-different-can-this-behavior-be-expected/335 "2023-01-23T14:47:36Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![ChristianHoffmann](https://avatars.discourse-cdn.com/v4/letter/c/a87d85/32.png) [@ChristianHoffmann](https://discuss.ampl.com/u/ChristianHoffmann)\
**Post date:** [January 23, 2023, 2:47pm UTC](https://discuss.ampl.com/t/solution-with-and-without-presolve-is-different-can-this-behavior-be-expected/335/1 "2023-01-23T14:47:36Z")

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Hello,

I have solved a simulation problem with AMPL (objective function = 1) with

```
presolve = on, CONOPT
presolve = off, CONOPT
presolve = on, IPOPT
presolve = off, IPOPT

```

In three cases, I obtain a solution that makes little sense to me. The combination with presolve off and CONOPT returns a solution I can understand.

Of course, it is possible that solvers return different solutions, but I was surprised to see that CONOPT returns different solutions depending on the presolve setting.

My question is: Is such a behavior expected? I have to point out that the optimization problem seems to be badly conditioned - just in case this might influence the observed behavior.

Should you need more information, I am happy to provide it.

Christian

---

<div class="post-metadata">

**Author:** ![4er](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.ampl.com/4er/32/530_2.png) [@4er](https://discuss.ampl.com/u/4er)\
**Post date:** [January 24, 2023, 1:22am UTC](https://discuss.ampl.com/t/solution-with-and-without-presolve-is-different-can-this-behavior-be-expected/335/2 "2023-01-24T01:22:24Z")

</div>

A response to this question appeared in [Google Groups](https://groups.google.com/g/ampl/c/w49RaOWYYh8), and the response is also mirrored in a [Discourse post](https://discuss.ampl.com/t/ampl-24485-solution-with-and-without-presolve-is-different-can-this-behavior-be-expected/327).